To keep the base position of a structural steel and a level adjusting bolt at an accurate position by fastening a pendulum with a small machine bolt in four directions of a cross jig and placing a specific-sized hole bored at the core of the cross jig into the base position of the structural steel or the head of an level adjusting bolt and aligning the pendulum with a ground marking.

A hole which allows the base position of a structural steel or the head of a level adjusting bolt to be placed is bored at the core of a cross jig 1. Then, a pendulum 2 is fastened with a small machine bolt 3 is four directions of the cross jig 1. At that time, the core hole is bored in such a fashion that its hole diameter may be larger than the diameter of the base position of the structural steel or the head of the level adjusting bolt by 0.1 to 0.3mm. The pendulum 2 is mounted in all directions at an accurate position of 90°. Furthermore, a screwless area of the small machine bolt 3 is made longer from 0.5 to 1mm than the thickness of the pendulum 2 where the hole in the cross jig 1 is placed in the base position of the structural steel and the head of the level adjusting bolt and the base position and the adjusting bolt are moved so that the pendulum may be aligned with the ground marking, thereby fastening the adjusting bolt wit the nut. This construction makes it possible to attain an accurate alignment.